The present invention relates to a display unit or optical indicator and, more specifically, to a pneumatic display unit having a mirror-coated part placed within, and which may be seen through, a transparent housing. A shutter part is provided for making a change in the light acting on the mirror part when the display unit is acted upon by a change in the pressure of a space in which pressure is to be monitored, for example, with respect to an increase in pressure. Such display units or indicators are used, for example, in control systems for displaying signals, such as control signals or feedback signals.
One purpose of the present invention is that of overcoming the shortcomings of known display units of this sort and of designing a display unit which may be cheaply produced, has a small overall size and may be used for very wide pressure ranges.
For effecting this purpose and other purposes, in the present invention, the display unit is so designed that, on the one hand, it has a hollow guide stem, whose front end to be seen is in the form of a cover made of transparent material, the hollow guide stem having within it a fixed-position or nonmovable inner part whose end nearest the cover is mirror-coated, while on the other hand there is a piston, able to be moved backwards and forwards axially within the hollow guide stem, the piston being loosely but drivingly joined up with a shutter part of a sleeve. The sleeve is placed round the fixed-position or nonmovable inner part and is guided within the guide stem. In this respect, the design may be such that the body of the nonmovable inner part takes the form of a tube having a return spring within it, one end of which rests on a joint between the tube and an end of the fixed inner part and the other end of which rests against an end wall of the sleeve nearest the piston. Furthermore, the end of the fixed inner part may take the form of a generally cylindrical base and a body of revolution becoming narrower in a direction away from the base towards a point, the body of revolution being mirror-coated and having a generatrix in the form of a concaved curve as for example a hyperbola, a parabola, part of an ellipse or the like.
The new display unit or indicator of the invention is marked by a small overall size and it may be used over a very wide pressure range. Furthermore, its moving parts are free-running and the cost of producing it is low because, to a large degree, the inner parts may be injection-molded. Furthermore, the parts may readily be put together and assembly undertaken by untrained workers, since it is hardly possible for there to be any loss of parts on putting the display unit together. Generally speaking, the display unit may be used for large control systems for which feedback signals and command signals have to be displayed all the time. Thus, an important useful effect of the invention in this respect is that the display unit takes up little space and a great number of them may be positioned on a relatively small support plate.
For reasons of production and assembly of the display units, a particularly useful effect is produced if the hollow guide stem is made up of two halves which are joined together, for example by a threaded connection one half terminating in a cover made of transparent material and the other half terminating in a cross-wall having a pipe connection thereon for facilitating a joining up with the space in which the pressure is to be monitored, such space then being joined up with the space within the hollow stem so that the command or feedback pressure signals may take effect within the hollow stem or cylinder through a pipe connection and may be displayed. For the purpose of making the design and assembly of the indicator simpler, it is possible, as a further development of the teaching of the invention, for the sleeve to be made up of two sleeve halves joined together end-to-end and having a different diameter. The sleeve halves are then joined together at a shoulder, the sleeve half with the larger diameter being nearer the mirror part on the tubular fixed inner part, while the other sleeve half is only a little greater in diameter than the tube. The sleeve half with the larger diameter has an inner diameter which is somewhat greater than the outer diameter of the tube of the fixed inner part, whose end, that is to say the mirror part, is best made somewhat greater in diameter (than the tube). In this respect, the design may be such that the hollow stem half with the cover made of transparent material has, towards the middle thereof, an inwardly extending shoulder, against which the shoulder between the two halves of the sleeve comes to rest in the ready-for-use condition of the sleeve. It is best for the stem half with the cover made of transparent material to have an air outlet opening in its middle part, generally at the same level as its shoulder. This air outlet opening is used for bleeding air from inside the guide stem.
A particularly simple operation is made possible if, for example, the fixed inner part and the sleeve are so placed in relation to each other that the sleeve in the starting position thereof is under the effect of a return spring to hold the sleeve away from the mirror part or end of the fixed inner part and when the display or indicator is acted upon by the pressure, answering for example to an increase in pressure, the sleeve will be placed so as to have covered or nearly completely covered the mirror part. For making assembly simpler, it is best for the stem half having the cover thereon to have at its front end, and more particularly at the point at which it is joined with the cover, a radially outwardly extending lip or shoulder. It is preferable for the stem half with the cover thereon to have a coil or other type compression spring encircling same and resting at one end against the free end face of the threadedly connected stem half. The lip-like broader part or shoulder on the end of the stem half with the cover is slipped through a hole in the wall in an outward direction, the lip-like wider part or flange on the stem half coming to rest against the outer face of the support wall. The compression spring takes effect on the other side of the support wall by reason of its other end resting against the end face of the other stem half. On fixing the unit to a support wall, the first step to be taken is for the one stem half to be slipped through the opening in the support wall until its lip comes to rest against the outer face of the support wall and then the sleeve and the fixed inner part within it together with its return spring are put into position from the outer end of the stem. Thereafter the transparent cover is put in position and then from the inner end the second stem half with the piston and the other inner parts is put in position. It will be seen from this that assembly has been made simpler and may readily be undertaken by untrained workers; furthermore loss of any of the parts of the unit on assembly would hardly be possible. It will at once be seen that assembly is responsible for a saving in time.